U.S. Pat. No. 4,779,994 discloses a thin film heat flux sensor and a method for manufacturing it. The manufacturing method has certain drawbacks which limit the sensor performance and restrict the range of applications. The manufacturing cost of the sensor is high because it is made by multiple stages of sputtering through shadow masks. Another drawback is the relatively low sensitivity of the sensor; heat flux is measured by measuring the temperature drop across a very small thermal resistance, and signals are of the order of a few microvolts per watt/cm.sup.2. In many applications for such sensors, heat flows to be measured are a small fraction of 1 watt/cm.sup.2, and thin film sensors cannot be used.
A heat flux sensor indicates the rate and direction of heat energy flow. When the measurements of such a sensor are combined with those of a temperature sensor at the same location, all of the following quantities are indicated:
(a) the present temperature; PA1 (b) the present heat flux; PA1 (c) the heat transfer coefficient; PA1 (d) the effective thermal capacity; PA1 (e) the projected rate of temperature change at the present heat flux; and PA1 (f) the projected rate of temperature change at any other value of heat flux. PA1 R.sub.m =effective series thermal resistance PA1 R.sub.h =series thermal resistance (conductive) of the HFT alone PA1 R.sub.c =thermal contact resistance between HFT and substrate PA1 R.sub.ms =total thermal surface resistance (convective and radiative) over HFT PA1 R.sub.s =total thermal resistance (convective and radiative) over surrounding area
In addition, the temperature and heat flux signals can be compared to detect drift or failure of either sensor.
Measurement of heat flux is critical to the understanding or control of many thermal systems. When both heat flux and temperature data are available for the same point on a surface, one can calculate such material properties as thermal resistance and thermal diffusivity. In situ measurement of heat flux is essential for the performance evaluation of insulative building materials. This is because it is often difficult or impossible to predict the installed performance of insulative materials from laboratory experiments.
Heat flux transducers (HFT's) have been used in building energy management applications since the 1950's. Methods for using them to evaluate the thermal performance of building materials are generally well understood. HFT's designed for surface mounting are inexpensive and easy to install. However, it is often difficult to acquire accurate, useful data with them.
The heat flux through a surface cannot be measured without some disturbance caused by insertion of the measuring device into the path of heat flow. The amount of change produced by the measuring device depends on many factors. These include; the contact resistance between the HFT and the test wall and other physical parameters such as surface emissivity, surface roughness and the thermal resistance of the HFT itself. These factors comprise the effective series thermal resistance, R.sub.m defined by EQU R.sub.m =R.sub.h +R.sub.c +(R.sub.ms -R.sub.s) (1)
Where:
The effective series resistance of a surface mounted HFT is the most important single factor affecting the error produced by disturbance of the measured heat flux. If R.sub.ms and R.sub.s are made approximately equal by matching the emissivity and the surface roughness of the HFT to corresponding values for the surrounding material, the effective series resistance is reduced to the sum of the HFT series thermal resistance and the thermal contact resistance. The series thermal resistance can vary widely in commercially available surface mounted HFT's; from about 0.002 m.sup.2 .degree.C./W to 0.1 m.sup.2 .degree.C./W. The thermal contact resistance is minimized by attaching the sensor to the surface with a very thin layer of high thermal conductivity adhesive.
For maximum utility in building energy management, a heat flux sensor should have a series thermal resistance of less than 1.times.10.sup.-4 m.sup.2 .degree.C./W. However, when the series thermal resistance of a thermopile type HFT is very low, its sensitivity may also be low because the low thermal resistance only produces a small temperature difference.
Another factor which affects the utility of a heat flux sensor in building energy management is shunting of heat flux around the sensor. Shunting can be significantly reduced by increasing the dimensions of a sensor. This solves the problem because the thermal resistance of the path around the sensor is directly proportional to the distance the heat travels. Large area HFT's can also provide more accurate data over non-homogeneous areas, such as across wall studs in framed buildings or on truss roofs. A large surface mounted HFT averages out the effects of such features. Unfortunately large HFT's are very expensive. According to a recent survey, the typical cost of a commercially available 12".times.12" HFT is over $600.00.
The copper conductors of printed circuit boards are produced by a number of processes. The most common of these is photoetching. In this process a board completely coated with copper and covered by a photopolymer is exposed to ultraviolet light through a negative transparency of the desired conductor pattern, solvent washed to remove the polymer where it has not been hardened by exposure, and then acid etched to expose the desired conductors.
A second process for producing printed circuit boards, known as the additive process, consists of a first step of electroless deposition of a very thin nickel layer representing the desired conductor pattern, followed by a second step of electrolytic deposition of the desired thickness of copper on the nickel conductors.
There is a lesser known third process for making printed circuit boards. In this process conductors are deposited as inks on an insulating substrate by screen printing. The ink traces are dried to a solid by rapid heating in a vapor reflow oven, then converted to metal by an elevated heat treatment. This process could be adapted to heat flux sensor manufacturing, if it could be used to deposit conductors of two different metals in an appropriate pattern.